Methods for producing monodispersed particles of barium titanate

Abstract

The present invention is a low-temperature controlled method for producing high-quality, ultrafine monodispersed nanocrystalline microsphere powders of barium titanate and other pure or composite oxide materials having particles ranging from nanosized to micronsized particles. The method of the subject invention comprises a two-stage process. The first stage produces high quality monodispersed hydrous titania microsphere particles prepared by homogeneous precipitation via dielectric tuning in alcohol-water mixed solutions of inorganic salts. Titanium tetrachloride is used as an inorganic salt precursor material. The second stage converts the pure hydrous titania microsphere particles into crystalline barium titanate microsphere powders via low-temperature, hydrothermal reactions.

Description

The present invention relates to a method for producing pure monodispersed microsphere powders of barium titanate. The present invention particularly relates to a method using mixed solvent synthesis for producing ultrafine monodispersed microsphere powders of pure materials such as titania and zirconia and further converting these pure materials using low temperature hydrothermal reactions to produce pure nanocrystalline microsphere powders of barium titanate, strontium titanate, lead titanate, zirconium titanate, titanium zirconate, lead zirconate, and others, wherein the powders comprise particles ranging from nanosized particles to micronsized particles.Oxide-based ceramics have many applications as structural or functional materials. Ultrafine, monodispersed oxide powders are important building-block materials for fabrication of advanced ceramics. The characteristics of powders, in terms of size shape / morphology, monodispersity, and microstructure, directly affect the quality o...

AI Technical Summary

Benefits of technology

Accordingly, it is an object of the present invention to provide an energy efficient, low-cost, low-temperature method for producing high-quality ultrafine monodispersed powders having microspherical particles of a pure material such as titania.

It yet another object of the present invention to provide an energy-efficient, low-cost method for producing high-quality ultrafine monodispersed powders of barium titanate, having particles ranging from nanosized particles to micronsized particles.

FIG. 6 shows the effect of conversion time on the BaTiO.sub.3 formation (at Ba / Ti=1), indicating that longer times favored BaTiO.sub.3 conversion.

Once the inorganic salt was dissolved in water as mentioned above and as described by Park et al. (J. Am. Ceram. Soc., 80 (3), 1997) incorporated herein by reference, then mixed with isopropanol. Isopropanol was chosen to the most favorable alcohol system to reduce the dielectric constant of particle-forming medium The volume ratio of alcohol to aqueous part (RH), initial concentration of titanium salt (TiCl.sub.4), incubation time, and incubation temperature were controlled to allow for the rapid formation of uniform well-dispersed microspheres of hydrous titania of a particular size, which were then used as a good-quality precursor for the formation of nanocrystalline barium titanate particles. It was found that a narrow RH ratio (i.e., an alcohol / aqueous volumetric ratio of 2 to 4) was required for optimal nucleation and growth of monodispersed titania particles. A 30-mL volume of the mixture containing TiCl.sub.4, HPC, water, and isopropanol was routinely kept in a 40-mL capped glass vial that was incubated in an oven. Unless otherwise indicated, the conditions for titania microsphere synthesis in this EXAMPLE of the present invention were: 0.2 M TiCl.sub.4, RH ratio of 2, 0.002 g / cm.sup.3 HPC, and incubation at 100.degree. C. for 24 hours in an oven. (Although the precipitation may have completed in a few hours, longer incubation ensured that the TiO.sub.2 microspheres were completely aged.) The white microsphere particles settled to the bottom of the glass vial after incubation and were resuspended easily by shaking the vial manually. The glass vial containing the particle suspension was cooled down to room temperature by immersion in tap water in a container. Immediately after cooling, the suspension was neutralized by titration using 5 N NH.sub.4 OH with constant stirring to chemically stabilize the microspheres. Without neutralization, the microspheres could redissolve with the addition of water during a later-stage washing step. Centrifugation and resuspension with deionized water (30 mL) were routinely repeated twice to rid the suspension of any impurities, such as chloride or ammonia, that might interfere with the X-ray diffraction (XRD) spectrum. The resulting washed titania suspension (containing 0.2 mol titanium equivalent) was kept in deionized water for hydrothermal conversion and synthesis of barium titanate.

To evaluate the stability of the barium titanate at high temperatures, the sample from the Ba / Ti=1 condition was also heated to 300, 600 and 750.degree. C. in HTXRD chamber for longer-time phase stability tests. The first three XRD scans were taken immediately after each other and then three more scans were taken 1 hour after each other. Since each scan required approximately 20 minutes, the total calcination time at each of the three temperatures was over 4 hours. The barium titanate peaks were not altered at these three temperatures, demonstrating the thermal stability of the sample.

Problems solved by technology

The resulting microstructure has not been sufficient for microelectronic applications due to a lack of uniform nanocrystalline barium titanate.

The resulting microstructure has a wide grain-size distribution, multiple phases, and is inevitably porous.

These characteristics result from inadequate existing ball-milling operations that can introduce such impurities as alumina, silica, sulfur, phosphorus, etc.

The lack of control over the physical and / or chemical characteristics of commercial barium titanate particles results in microstructural variations that lead to poor electrical property optimization and reproducibility.

However, these techniques are not without some serious shortcomings.

The hydrothermal methods using titania gel or commercial titania particles are effective (kinetics and composition wise) in producing submicron barium titanate, however, synthesis of very uniform (also dispersed) microspheres has not been achieved and thus needs more development.

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